1. Technical Field
The invention relates to a process for recovering hydrocarbons from a subterranean formation, and more particularly to a hydrocarbon recovery process using a permeability-reducing agent.
2. Background Information
The utility of crosslinked polymer gels as permeability-reducing agents in subterranean hydrocarbon-bearing formations to facilitate hydrocarbon recovery has long been known in the art. U.S. Pat. No. 3,762,476 to Gall is representative of conventional teaching in its time. Gall discloses a conformance improvement treatment process, wherein a permeability-reducing crosslinked polymer gel is formed in situ by sequential injection of the gel components. An aqueous slug containing a crosslinkable water-soluble polymer is injected into the subterranean treatment region followed by an aqueous slug containing a crosslinking agent made up of a polyvalent metal cation complexed with a retarding anion. Useful polyvalent metal cations listed in Gall include iron(II), iron(III), aluminum(III), chromium(III), calcium(II), and magnesium(II). Useful retarding anions complexed with the cation include citrate, phosphate, acetate, nitrilotriacetate, and tartrate.
Sequential injection of the gel components as separate and distinct slugs into the treatment region is essential to treatment processes such as Gall insofar as surface contacting of the polymer and crosslinking agent was believed to cause premature gelation of the gel components prior to reaching the treatment region. If prematurely formed at the surface or in the well bore, placement of the gels in the treatment region is difficult, if not impossible, to achieve. Consequently, sequential injection of the gel components in concept avoids premature gelation by delaying contacting of the gel components until they are displaced out into the treatment region of the formation.
In situ contacting of the polymer and crosslinking agent as required by Gall, nevertheless, is operationally unattractive in many hydrocarbon recovery applications because of the difficulty in achieving adequate mixing of the gel components in situ. Without adequate mixing, the gels of Gall are poorly formed, resulting in weak and unstable gels that perform ineffectively as permeability-reducing agents.
U.S. Pat. No. 3,926,258 to Hessert et al. offers an operational alternative to conformance improvement treatment processes such as Gall that effectuate gel formation by sequential injection of the gel components into the treatment region of the formation. The process of Hessert et al. enables mixing of the gel components at the surface to form a gelation solution that can be injected into the desired treatment region of the formation as a single slug. Thus, Hessert et al. overcomes the inherent operational limitations of processes that require in situ mixing of the gel components. Hessert et al. avoids premature gelation of the solution by specifying and carefully controlling the sequence and form in which the gel components are mixed at the surface to generate the gelation solution.
In accordance with Hessert et al., a gelation solution is prepared by initially forming an aqueous solution of a crosslinkable polymer and a multivalent metal cation such as chromium(VI). The metal cation is at an elevated, but reduceable, valence state at which the metal cation is substantially inert with respect to the polymer. A complexing agent is then added to the solution followed by a reducing agent. The reducing agent reduces the valence state of the metal cation to a valence state at which the metal cation is reactive with the polymer. The complexing agent, however, delays the crosslinking reaction between the polymer and reduced metal cation by complexing with the metal cation until the resulting gelation solution can be placed in the treatment region. Placement of the gelation solution, nevertheless, cannot be delayed inordinately once the reducing agent is added to the solution because the polymer competes with the complexes for the reduced metal cations.
Like Hessert et al., U.S. Pat. No. 4,683,949 to Sydansk et al. discloses a conformance improvement treatment whereby a gelation solution is formed in its entirety at the surface before the solution is injected as a homogeneous slug into a desired treatment region. Sydansk et al. is specifically predicated on the finding that gelation solutions prepared entirely at the surface from a chromium(III)/carboxylate complex crosslinking agent and a crosslinkable polymer produce gels that are uniquely stable and have highly predictable gel strengths and gelation times. The gels are capable of practical long-term placement in subterranean treatment regions, even in hostile environments. Unlike Hessert et al., however, Sydansk et al. is not specific with respect to the method of preparing the gelation solution at the surface. Sydansk et al. teaches alternative methods of preparing the gelation solution, but ascribes no criticality to the particular sequence of preparation. Sydansk et al. is preferentially directed to the ultimate composition of the gelation solution at the surface rather than the method of preparation.
U.S. Pat. Nos. 5,131,469 and 5,143,958 to Lockhart et al. disclose a preferential embodiment for preparing complex crosslinking agents of the type described by Sydansk et al. in conjunction with preparing gelation solutions at the surface having utility in conformance improvement treatments. According to the preferred embodiment of the Lockhart et al. patents, chromium(III)/carboxylate complex crosslinking agents are prepared by adding an inorganic chromium(III) salt and an acid or salt form of a designated carboxylate ligand directly to an aqueous solution of a crosslinkable polymer at the surface. In doing so, the complex crosslinking agent and gelation solution are generated simultaneously for injection into the desired treatment region. According to an alternate embodiment of the Lockhart et al. patents, the complex crosslinking agent is preformed ahead of the gelation solution by combining the inorganic chromium(III) salt and the acid or salt form of the carboxylate ligand absent the polymer. The polymer is subsequently added to the preformed complex crosslinking agent in solution to form the gelation solution.
Notwithstanding the teaching of the Hessert et al. and Lockhart et al. patents, a need exists for additional means of preparing chromium(III)/carboxylate complex crosslinking agents and resultant gelation solutions insofar as it may not always be desirable to form the complex crosslinking agent in the presence of the polymer as required by Hessert et al. Furthermore, the use of chromium(VI) in field applications may be undesirable because chromium(VI) is a regulated material having potential toxicity to the environment. It is also apparent that the inorganic chromium(III) salt starting material required by the Lockhart et al. patents may be undesirable for preforming or otherwise preparing complex crosslinking agents because the inorganic chromium(III) salts taught therein are not always abundantly available in the field in the required quantities and the inorganic chromium(III) salts are more highly regulated and more corrosive than organic chromium(III) salts.
It is, therefore, an object of the present invention to provide a gelation solution composition at the surface that matures into a crosslinked polymer gel for permeability reduction of a subterranean hydrocarbon-bearing formation. It is another object of the present invention to provide a process for preparing the gelation solution at the surface. It is a further object of the present invention to provide such a process for preparing the gelation solution utilizing starting materials alternate those taught by the prior art. In particular, it is an object of the present invention to provide such a process for preparing the gelation solution utilizing starting materials less corrosive than those taught by the prior art. It is yet another object of the present invention to provide such a process for preparing the gelation solution utilizing starting materials that are for the most part readily available in the field.